1. Field of the Invention
This invention relates to antennas and, more particularly, to a loop antenna of moderate electrical size having an omnidirectional far-field pattern similar to that of an electrically-small loop.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Electric and magnetic dipole antennas having ideal omnidirectional patterns are very useful for design, operation and testing of various electromagnetic systems. For example, electric dipoles are often used to make so-called site attenuation measurements and for characterizing test sites used in testing antenna systems. Site attenuation measurements are essentially insertion loss measurements made with two precision dipoles carefully positioned a fixed distance apart. The deviation in insertion loss between the two dipoles as compared with the insertion loss between the dipoles in xe2x80x9cfree spacexe2x80x9d (actually, a reference site) gives an indication of the quality of a test site. However, the electric dipoles can mask some problems with a site in that they do not radiate in all directions; they exhibit radiation nulls located on their dipole axes. A magnetic dipole also has radiation nulls on its dipole axis (a perpendicular running through the center of the loop). However, by using two electric (horizontal and vertical) and two magnetic dipoles (horizontal and vertical), masking effects of the nulls may be overcome. An xe2x80x9comnidirectionalxe2x80x9d or xe2x80x9cisotropicxe2x80x9d pattern as used herein refers to a pattern having constant field amplitude with direction within a two-dimensional plane perpendicular to the axis of an electric dipole, or, in the case of a magnetic dipole loop, the plane containing the loop. In other words, the dipole cannot be literally omnidirectional because of its radiation nulls, but it is desirable that the dipole be omnidirectional in the plane perpendicular to the direction containing the nulls. Dipoles having such idealized patterns are needed to obtain accurate characterization of test sites.
Isotropic patterns are also desirable in mobile communications systems, in which the direction from which an incoming signal comes may be constantly changing. The large amount of scattering and reflection encountered in typical mobile communications systems makes it desirable to employ antennas with different polarizations, so that the chance of detecting a signal having an arbitrary polarization is increased. An electrically-small magnetic loop dipole radiates a dipolar pattern which is orthogonal to that of an electric dipole. Thus, such an antenna is useful when pattern diversity is required.
Electric (linear wire) and magnetic (loop) dipoles exhibit omnidirectional far-field patterns when used at frequencies for which they are electrically-small, or for which the physical size of the antenna is small compared to the wavelength of radiation. For the purposes of this disclosure, xe2x80x9celectrically-smallxe2x80x9d refers to an antenna having its largest dimension smaller than about {fraction (1/10)} of a wavelength. Electric dipoles having omnidirectional patterns may be realized fairly easily. A wire dipole exhibits a fundamental series resonance (the linear or wire dipole exhibits a minimum susceptance input admittance; it is an open circuit at DC) when it is slightly less than one-half wavelength long. At this point the input impedance to the dipole is about 73-80 Ohms and thus is very nearly intrinsically matched to a 50 Ohm source. Furthermore, the pattern of the so-called half-wave dipole differs only slightly from that of an electrically-small dipole. Patterns of ideal (electrically-small) and half-wave electric dipoles are discussed further in pages 200-222 of Antennas by John D. Kraus (McGraw-Hill, 1988, hereinafter xe2x80x9cKrausxe2x80x9d), which pages are hereby incorporated by reference as if fully set forth herein.
Practical realization of a magnetic dipole having an omnidirectional pattern, on the other hand, is more difficult. A single-turn loop antenna, or magnetic dipole, exhibits its fundamental parallel resonance (a loop exhibits a minimum reactance input impedance; it is a short circuit at DC) at a frequency when it is very nearly one wavelength in circumference. However, the pattern of a self-resonant loop is completely different from that of an electrically-small loop, and is not omnidirectional. In fact, the maximum field amplitude is not even in the plane of the loop, as it is for the electrically-small loop. Patterns of magnetic dipoles of various electrical sizes are discussed further in pages 238-255 of Kraus, which pages are hereby incorporated by reference as if fully set forth herein.
A classical magnetic dipole therefore needs to be electrically-small to produce an omnidirectional pattern. There are several reasons, however, for using an antenna which is not electrically-small. An electrically-small loop has a very small radiation resistance and very high radiation Q. The high radiation Q corresponds to narrowband radiation characteristics. Furthermore, it is much easier to match an antenna of moderate electrical size to a 50-ohm source (50-ohm sources are most common, and other typical impedances, such as 75 ohms, are also relatively large). In a metrology antenna, the matching network can contribute significantly to measurement uncertainty. This is because of necessarily non-zero tolerances in matching components and because of temperature sensitivity of the matching components. In addition, at higher UHF frequencies and above it becomes difficult to implement an electrically-small antenna with precision. This is because the short wavelength requires a very physically-small antenna with the attendant tight dimensional tolerances. That is, the dimensional tolerances are related to the wavelength and the overall size of the antenna.
Finally, while in principle it is possible to scale any linear electromagnetic device, some details cannot easily be scaled in practice. For example, connectors and coaxial transmission lines are commercially available only in specific sizes and geometries. It is not at all worthwhile to design and manufacture custom connectors for a specific antenna. Furthermore, if custom connectors were developed, adapters to allow interconnection with industry-standard connectors would also be required. Thus, it is best if designs can employ standard coaxial connectors such as SMA connectors. If, for example, it were necessary to implement an electrically-small antenna at 2450 MHz, the antenna would be roughly the same size as the SMA connector. Obviously, in this case, the external geometry of the connector would influence the radiation pattern of the antenna. In most cases, it is useful if the external geometry of the connector and feed transmission line have minimal influence on the operation of the antenna.
Further discussion of the use of omnidirectional antennas and problems with electrically-small loops is included in U.S. Pat. No. 5,751,252 to Phillips (hereinafter xe2x80x9cPhillipsxe2x80x9d), which is hereby incorporated by reference as if fully set forth herein. An approach described in Phillips to making an omnidirectional loop antenna involves xe2x80x9cbreakingxe2x80x9d the loop at a point opposite the feed point of the loop, and bridging the break with a capacitive element. By effectively open-circuiting the loop at what would be the maximum current point of the (unbroken) loop, this approach lowers the overall current variation around the loop, resulting in a more omnidirectional pattern. The diameter of the loop described in Phillips is {fraction (1/7)} of a wavelength, which although larger than a classical electrically-small loop, may still be undesirably small, particularly for operation at higher frequencies (e.g., greater than one GHz). There further appears to be no indication in Phillips of how the small capacitor values needed (0.7 pF at 800 MHz) are to be realized with the precision necessary for a metrology grade antenna.
Another approach is to simulate a large loop using four small loops connected in parallel across a coaxial line. This xe2x80x9ccloverleafxe2x80x9d antenna is described on pages 731-732 of Kraus, which are hereby incorporated by reference herein. The cloverleaf antenna is a broadcasting antenna, and is not believed to exhibit sufficient omnidirectional uniformity for metrology applications. Driving of the small loops is further believed to result in a smaller bandwidth than would be realized by an actual large loop antenna.
It would therefore be desirable to develop a magnetic dipole antenna of moderate electrical size having an omnidirectional far-field pattern. The antenna should also be readily implemented and exhibit a bandwidth commensurate with its overall electrical size.
The problems outlined above are in large part addressed with an antenna including a conductive loop having multiple feed points spaced around the loop. The loop is opened at each feed point, and the feed points are preferably spaced evenly around the circumference of the loop. Four feed points spaced at 90 degree intervals are used in a currently preferred embodiment, but two, three or higher numbers of feed points may also be used in some embodiments. A respective feed line may be coupled to each of the feed points, and a structure for maintaining the portions of the discontinuous loop in position may be included. In an embodiment, the feed lines are balanced lines. A matching element may be included at each feed point.
In a currently preferred embodiment, the feeds are implemented using shielded lines, and the resulting loop antenna can be viewed as a multiply-fed shielded loop antenna. This shielded loop embodiment may be implemented by placing insulated feed wires into channels formed within a conductive structure. The channel therefore forms the outer conductor, or shield, for a coaxial line having the feed wire as an inner conductor. The conductive structure includes an outer loop and radial arms through which the feed lines are routed to a shunt connection at the center of the loop. The radial arms may be joined at the shunt connection, thereby providing mechanical support to maintain the positions of the portions of the discontinuous outer loop. The radial arms may meet the loop at positions equidistant between adjacent feed points. Each feed line may be routed from the central shunt connection out to the loop, then turn and follow the loop circumference to reach its respective feed point (gap in the loop). In an embodiment, the feed line is continued past the feed point to form an open-circuited transmission-line stub. Such a stub forms a series capacitance which may be used for impedance matching at the feed point.
A kit including one or more components of the shielded loop antenna described above may include a conductive structure in the form of a loop having multiple arms extending radially from the loop toward a point at the center of the area surrounded by the loop. The loop may include multiple portions separated by feed gaps. The conductive structure may include a respective channel extending from each feed gap and toward the point at the center of the area surrounded by the loop, where each channel is adapted to hold an insulated feed line. The channel may further include an extension past its respective feed gap, where the extension is adapted to hold a portion of insulated feed line forming an open-circuited transmission line stub. In an embodiment, the conductive structure may include two similar structure portions adapted to be fastened together after placement of the insulated feed lines between them. Each channel may be formed from a respective groove in at least one of these structure portions.
The kit may further include a conductive stem structure adapted for attachment to the arms of the conductive structure, where the conductive stem structure includes a conductive tube. The stem structure and the conductive structure are adapted such that an axis directed perpendicular to the plane of the loop and through the point at the center of the area surrounded by the loop is directed along the interior of the conductive tube when the stem structure is attached to the conductive structure. In a further embodiment, the kit may include insulated feed line adapted to be arranged within each of the channels in the conductive structure. The feed line may be adapted such that the characteristic impedance of the shielded line formed by arranging the feed line within the channel matches an impedance of the loop seen at the feed gap corresponding to the feed line. The kit may further include an insulated stem conductor line adapted to be arranged within the conductive tube of the conductive stem structure, and electrically coupled to a shunt connection of the feed lines arranged within all of the channels. The stem conductor line may be adapted such that its characteristic impedance when arranged within the stem structure causes a quarter-wave transformation of the impedance at the shunt connection to the impedance of a source or receiver to be coupled to the antenna.
The impedance (including matching elements) at each feed point preferably matches the characteristic impedance of its respective feed line. The impedance at the shunt connection of the feed lines may be matched to the source impedance using a quarter-wave transformer. The transformer may be included within a supporting stem for the antenna arranged along the perpendicular axis running through the center of the loop. This feed orientation is in the direction of a null in the radiation pattern, and therefore minimizes interference between the feed and the pattern.
Use of multiple feeds spaced around a loop antenna, as described herein, is advantageous in providing an omnidirectional pattern from a loop of moderate electrical size. Each loop portion between adjacent feed lines is relatively small electrically, and exhibits a substantially constant current distribution. The entire loop therefore has a constant current distribution, resulting in an omnidirectional pattern. The relatively large electrical size of the entire loop provides a large operational bandwidth and high radiation efficiency. In an embodiment, the loop diameter is approximately one-quarter of the operating wavelength, and the arc length of each separately-fed loop portion is less than about one-quarter of the operating wavelength. In the case of the shielded loop embodiment, the antenna may be implemented using precision machining techniques, allowing good control of critical dimensions.
In an embodiment of a method for forming an antenna, multiple feed points may be spaced apart around a conductive loop, and a respective feed line coupled to each of the feed points. The circumference of the loop divided by the number of feed points may be less than about a quarter of the operating wavelength of the antenna. In an embodiment, the feed lines may be shielded lines connected together at a shunt connection. An impedance at the shunt connection may be matched to that of a source for the antenna using a transformer. In an embodiment, the transformer is a quarter-wave transformer.